The present invention relates to screwdrivers, more specifically, to screwdrivers for bone screws.
A two part screwdriver with a grip part can be comprised of a driver blade and a chuck. The driver blade features a tip and is adapted for insertion into the grip part and is secured against rotation and unintentional removal relative to the grip part by means of a locking device. The chuck retains a screw which meshes with the blade tip, encloses the driver blade and allows displacement of the driver blade in the longitudinal direction. The displacement of the chuck is limited by a stop device when the chuck reaches a retaining position. The chuck possesses collets biased in the direction of the driver blade and has claws on the ends of the collets.
A three part screwdriver with a grip part can be comprised of a driver blade, a chuck and a sleeve. The driver blade features a tip and is adapted for insertion into the grip part and is secured against rotation and unintentional removal relative to the grip part by means of a locking device. The chuck retains a screw which meshes with the blade tip, encloses the driver blade and allows displacement of the driver blade in the longitudinal direction. The displacement of the chuck is limited by a stop device when the chuck reaches a retaining position. The chuck possesses collets biased away from the driver blade and has claws on the ends of the collets. A sleeve encloses the chuck, allows longitudinal displacement of the chuck and forces the outwardly biased collets toward the driver blade.
A screwdriver according to the initially cited two-part, inward biased, design is known from U.S. Pat. No. 1,441,956. Provided on this screwdriver is, cross-sectionally, an essentially quadratic driver blade, with a tip, which is surrounded by a longitudinally slidable chuck. For retaining a screw, the chuck possesses collets which are biased inwardly toward the driver blade, with claws on their ends. Provided as stop device for limiting the displacement of the chuck are four paired and opposed roundish grooves recessed in corresponding side surfaces of the chuck as well as two opposed projections pointing in the direction of the driver blade. Furthermore, the chuck is provided with a slot-shaped recess which is engaged by a pin arranged on the driver blade. The length of the recess matches the spacing of the groove pairs. The arrangement of the recess and the pin is such that, as the projections engage mutually opposed groove pairs, the pin strikes one end of the recess.
For grabbing a screw, the chuck is disposed in its front limiting position, in which the projections engage the grooves in the driver blade near the blade tip. A screw is insertable between the claws protruding beyond the blade tip when the collets are forced open. The screw is then retained by the inward bias of the collets. To mesh the screw with the blade tip, the chuck is retractable in the direction of the blade tip until the projections reach their retaining positions in the grooves pointing away from the blade tip. Thus, there are two defined positions of the chuck relative to the driver blade which allow easy adjustment.
The removal and restoration of the pin of this design make assembly and disassembly unsuitably expensive. This two-part screwdriver has the additional disadvantage that, as the chuck is being displaced between its retaining position and its front limit position, the projections run onto a section of the driver blade that has an enlarged cross section thereby reducing the retaining tension of the claws. As a result of the reduced tension, a screw can unintentionally drop-out relatively easily, for instance, when touching a side wall, under congested conditions. Moreover, once a screw has been placed, there is the risk of injuring sensitive biological tissue by bumping into it if the lateral forces necessary for chuck release are exerted under congested space conditions.
Known from DE 35 39 502 C1 is a screwdriver according to the initially cited three-part, outwardly biased, design. In this prior three-part screwdriver there is a driver blade surrounded by a longitudinally displaceable chuck which, in its end area near the tip, features outwardly biased collets with claws for retaining a screw. The collets are outwardly biased due to the relatively high outward spring tension of the collets. The chuck is surrounded by a longitudinally slidable retaining sleeve. By sliding the retaining sleeve toward the blade tip, the outwardly biased claws are longitudinally entrained by the chuck beyond the tip of the driver blade and are pressed inward to engage the screw head by the relative movement of the retaining sleeve over the collets. The movement of the retaining sleeve is a relatively energy-intensive action. To prevent the claws from sliding beyond the screw head, the driver blade is equipped with an annular shoulder of increased circumference which strikes a stop shoulder of the chuck at the level of the screw head when the claws reach a retaining position thereby preventing further advancement of the chuck.
To preclude an unintended displacement of the chuck in response to gravitational forces on the driver blade, for one, and of the retaining sleeve on the chuck for another, recesses and/or longitudinal slots with sections biased in the direction of the driver blade need to be provided in the chuck and retaining sleeve areas near the grip part. The recesses and/or slots are provided so that, due to the elevated frictional forces created by the sections bearing under bias, a displacement will take place only when intentionally actuated by the user. The defined adjustment of the friction forces makes fabrication relatively expensive. Additionally, necessary corrections, for example, due to damage in the screwdriver assembly attributable to the complex interaction of the various forces, are only possible with difficulty. Furthermore, due to the high outward bias of the collets, the user must exert relatively great forces to assemble the screw driver, and, during disassembly for cleaning, the risk of injury is undesirably high due to the numerous sharp edges.
In order for the chuck to be reliably advanced to its retaining position as the retaining sleeve is displaced toward the blade tip, the collets must expand relatively far outward, even when highly biased. This creates disadvantages not only during the assembly of the screw driver but also when the assembly is used in congested spaces and can result in injuries to biological tissues.
Known from CH-PS 369,416 is another three-part, outwardly biased, screwdriver that features a chuck surrounding a driver blade and having collets that are fitted on their ends with outwardly bent open claws. The collets are surrounded by a retaining sleeve for forcing the claws towards the driver blade in the area of a blade tip. The retaining sleeve possesses an inhibitory spring which is bent inward in the direction of the chuck and serves to enhance the frictional resistance between the chuck and the retaining sleeve. In this latter three-part screwdriver, the chuck and the retaining sleeve are independently displaceable by means of dish grips, however, defined positions to facilitate reliable operation are not provided. Instead, the user must take care that the chuck is correctly positioned relative to the driver blade and that advancing the retaining sleeve does not allow a retained screw to come off the driver blade. This is an extremely cumbersome process and is nearly impossible where the visibility of the screwing site is poor or nonexistent.
Known from DE 31 10 776 A1 and U.S. Pat. No. 855,280 are screwdrivers featuring driver blades surrounded by a chuck with a slip-over retaining sleeve. Such screwdrivers, while provided with defined positions, are nonetheless costly to assemble and disassemble. In the case of DE 31 10 776 A1, defined positions are achieved with expensive toggle lever mechanisms arranged on the retaining sleeves which require spring-supported toggle levers to engage recesses in the chucks.
Known from DE 84 03 371 U1 is a telescoping support where an interior tube is movable relative to an exterior tube by means of a tapped ring which is rotatable on an external threading of the internal tube and bears on one end of the external tube. To prevent the internal tube from slipping out of the external tube, a tongue type locking member in the form of a leaf spring, arranged on the external tube, is biased in the direction of the internal tube and engages in its limit position a groove recessed in the internal tube, thereby blocking a further displacement of the internal tube relative to the external tube.